Chrome ore refractories



Patented Apr. 20, 1937 PATENT OFFICE onnoma oar: REFRACTORIES Frederic A. Harvey and Raymond E. Birch, Pittsburgh, Pa., assignors to Harbison-Walker Refractories Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application July 10, 1935, Serial 20 Claims. (01. loo- 9) This invention relates to improvements in chrome ore refractories, and it is among its more important objects to provide chrome ore refractories of improved character, which possess desirable mechanical properties, such as cold crushing strength, modulus of rupture, and resistance to load at elevated temperatures, which do not undergo substantial volume change in burning or in reheating, which possess advantageous weight characteristics, and which are relatively cheap to make, retain desirable properties of chrome ore as a refractory material, and are applicable to the production of both formed articles and also refractory compositions for forming rammed linings, as mortar, and the like.

Chrome ore-is a desirable refractory material for many purposes because it is substantially neutral chemically and therefore is adapted for use under both acid and basic conditions, and further because .it is a cheap material. Also, it is one of the more refractory materials among those commonly used for these purposes. Chrome ore as a refractory is attended by certain disadvantages. For instance, refractories made solely or chiefly from chrome ore tend to show relatively low softening temperatures; for that reason their use has been limited to temperatures appreciably below the melting point of chrome ore, and at the same time it has been necessary to restrict the loads applied to the refractories. Likewise, brick made from chrome ore alone are quite heavy, and they undergo a substantial shrinkage, both during burning and upon reheating, to such an extent that the total shrinkage may be of the order of several-per cent.

The present invention is predicated on our discovery that refractories having the advantages of chrome ore but in which its disadvantages are repressed may be produced from 40 chrome ore, olivine and magnesia-rich material in which the chrome ore constitutes more than about 50 per cent, and suitably 50 to 80 per cent, of the refractory, the olivine at least 10 per cent, and the magnesia-rich material in amounts sub- 5 stantially between one-fourth to four times the amount of olivine in the refractory. In other words, olivine and magnesia-rich material are present between the ratios 1:4 to 4:1.

In such chrome ore compositionscontaining 50 olivine and magnesia between the limits stated we have found that not only are beneficial qualities of chrome ore retained to a high degree, but there are obtained refractories characterized by possessing better resistance to load at elevated. temperatures than refractories made from chrome ore alone. That is, the refractories will sustain a given load at higher temperatures than will chrome ore. For instance, brick made from, a high-grade chrome ore failed under a 25 pound per square inch load at a temperatureof about 5 2500 F. Brick made in accordance with this invention from the same chrome ore did not fall under the same load at temperatures below 2775 F., and in some of our'tests the brick showed only slight subsidence, without failure, under that 10 load at temperatures as high as 2910 F. The refractories made in accordance with the invention are characterized also by other desirable properties which will be referred to hereinafter.

The chrome ore referred to is a spinel mineral, 15 i. e., it comprises grains of mineral of the spinel type: ROB/203. Ideally, chrome ore would consist of grains of pure chromite spinel, FeQCrzOa Actually, however, the chrome ore grains are mixtures of isomorphous spinel minerals, such as 20 Fe0.Cr2O3, MgO.Al2O3 and MgO.Cr2O3 and perhaps others, and in chrome oresfrom various sources the amounts and proportions of these spinels vary considerably. Experience has shown, however, that notwithstanding such variations refractories made from the various chrome ores are substantially alike in behavior. Hence, considered broadly our invention contemplates the provision of refractories comprising from about 50 to about 80 per cent of material containing or 30 consisting of refractory spinel material of the character specified. For ease of reference and because it is a satisfactory refractory, readily available and a quite cheap member of that group, the invention will be described'with particular reference to chrome ore, it being understood that suitable refractory grades are employed.

Dead-burned, sintered or fused magnesite' are the preferred examples of magnesia-rich material, although other burned minerals high in 40 magnesia content, such as brucite, or magnesia produced by chemical methods may be'used, the latter being especially suitable where it is desired to provide the magnesia in finely divided form. Because magnesite represents the preferred em- 5 bodiment of this component of the refractories it will be referred to hereinafter for brevity of reference. The olivines used in the practice of the invention are those of the magnesium-iron type' which are high in content of magnesium 'ortho silicate. Suitable rocks are the dunites and per-; 7 idotites. The iron content of the olivine should; for most purposes, be relatively low, and advantageously it should not exceed about 10 per cent calculated as FeO. In the practice of the invenduced to finely divided form, and most advantion satisfactory results may be obtained with a dunite containing about 40.6 per cent of SiOz, 47.6 per cent of MgO, 7.5 per cent of FeO, 1.1 per 5 cent of AlaQ. 0.8 per cent of CraOa, 0.1 per cent of CaO, and an ignition loss of 1.2 per cent.

The most highly serpentinized and most highly steatitized ollvines are, in general, not suitable for the purposes of the present invention, nor other olivine minerals containing more than minor amounts of alteration products. However, we have successfully used olivines containing as much as 10 to per cent of talc-plus-serpentine, and depending upon the standards of refractoriness 15 which the products must meet, it may be possible for some purposes to use olivine-rich minerals containing somewhat greater percentages of these and other hydrous alteration products. As exemplifying compositions typical of the present invention, refractories may be made from the three essential constituents in proportions as follows:

A B C -D Chrome ore.-- 50 50 80 80 Magnesite 10 10 5 Olivine 10 40 10 15 In general, it is desirable for purposes of economy to relate the olivine and magnesite in such manner that the olivine is the preponderant ofthese two constituents. Because it comprises chiefly magnesium orthosilicate, olivine is of highly refractory character, and it is ordinarily cheaper than burned magnesite of good refractory quality. Therefore, the cost of the refractories will be reduced by using as much olivine as possible, although in some instances it may be desirable to have the magnesite in excess of the olivine, refractory A being an example of such a composition.

In the practice of the invention the chrome ore, olivine and magnesite, suitably grain sized, are intimately mixed in accordance with desired practice. The materials may be ground together,

or they may be ground separately and subsequently mixed after appropriate grain sizing.

a The particular grain size and relation of grain sizes of the materials do not appear to be critical, and they may be varied to provide particular properties, such as density and porosity, in the resultant refractory in accordance with relations known to those skilled in the art.

In most instances, however, the properties of the refractories are improved bycorrelating the grain size of the constituents so that the refractory formed thereof has maximum density. This may be accomplished by intermittent sizing of the constituents, which, for example, comprises 1 grinding so that 45 to 65 per cent of the materials pass a 4-mesh screen and are held in a 20- mesh screen while 25 to 55 per cent of the materials pass a 48-mesh screen. The intermediate grain sizes are suppressed as by screening out or holding to a minor percentage by proper control of the grinding operation. Preferably at least one of the constituents olivine and magnesite is retageously a large proportion of both of these constituents should be finely divided. One reason for this is that finely dividing these materials makes it possible for the magnesia of the magnesite to convert siliceous impurities in the chrome ore and hydrous silicates and similar impurities of low refractoriness present in the olivine to highly refractory constituents, such as to magnesium orthosilicate (2MgO.SiO2) and to magnesitoferrite (MgO.Fea0a). As exemplifying grain sizes which we have found to be suitable, the chrome ore may, by way of example, be ground to pass about 4 mesh, and the magnesite and olivine to pass 70 mesh.

The invention also contemplates graded sizing of the constituents and because commercial production methods are best adapted to graded sizing ordinarily it is employed. By graded sizing is meant that which results from a usual grinding operation, for example in an ordinary drypan when carried on for a determined time interval or until the material is fine enough to pass from the grinder by way of slotted plates or other openings of definite size. The material ground must, of course, be of a size to best serve its intended purpose and, by way of example, all the material will pass through a 4-mesh screen. Certain percentages of the material will also pass through screens of mesh of graduated greater fineness and a final percentage through a relatively fine screen. Ordinarily by reducing the grinding interval or by other means common in the art and differing with the type of crushing or grinding apparatus the percentage of given size fractions can be varied and the sizing controlled. A typical example of graded sizing as used in the present invention comprises about 20 to 30 per cent of the materials passing about 4 and being held on about 10 mesh, about 15 to 25 per cent passing about 10 and being held on about 28-mesh, about 10 to 20 per cent passing about 28 and being held on about 65 mesh and about 30 to 40 per cent of the materials passing about 65 to mesh. The materials may be ground separately or collectively to the graded size. However, as above explained with relation to intermittent sizing, the olivine and magnesite are preferably separately ground and to a finer grade than the chrome ore to assist in the chemical conversion .of impurities to refractory constituents. Broadly so long as the mixed batch ready for molding or the like contains the graded or intermittent grain sizes the requirements of the invention are fulfilled.

If the refractory is to be used as a lining or ramming mix, or as a mortar, or for similar purposes, it is necessary only to produce an intimate mixture of the three materials in appropriate proportions. Where shaped refractories are to be made the materials are mixed in a suitable mixer and tempered with water or other suitable liquid according to customary practice, for which reason mixing in an ordinary wet pan may be applied.

The prepared mixture is then shaped, advantageously under high pressure, and the bricks then dried and burned.

To illustrate the benefits derived from the practice of the inventiomreference may be made to \tests of brick of the composition C given hereinabove. These were made from a chrome ore containing 17.8 per cent of MgO, 32.7 per cent of CizOa, 14.5 per cent of iron calculated as FeO, 29.5 per cent of A1202, 4.5 percent of S102, and

the balance various impurities in minor amounts.

The magnesite used contained 92.4 per cent of MgO, 2.2 percent of CaO, 2.3 per cent of RaOs, 1.8 per cent of S102, and an ignition loss of 0.9 per cent. The olivine contained 49 per cent of MgO,

41 percent of 810:, 7.7 per cent of iron calmllated as FeO, 0.8 percent of A:, 0.3 per cent of CraOs. and an ignition loss of 0.8 per cent. The chrome ore was crushed to pass a 4-mesh screen, and the magnesite and olivine were ground to pass a '10- mesh screen. The materials were mixed in a proportion of 80 per cent of chrome ore and 10 per cent each of ma nesite and olivine, the batch being tempered with water. Standard size bricks were formed therefrom, and they were dried and then burned to about 2600" F. Tested under a load of 25 pounds per s uare inch, the brick did not fail until a temperature of approximately 2775" F. was reached. The bricks weighed but 164 pounds per cubic foot, as compared with 196 pounds per cubic foot for brick made wholly from the same chrome ore. They exhibited a cold crushing strength of 1360 pounds in burning and in reheating, although not to such an extent as to constitute an undesirable characteristic.

In a further test brick. were made from the same materials in the proportions of 60 per cent of chrome ore and 20 per cent each of magnesite and olivine. These'bricks were made and tested in the same manner as in the foregoing example.

They possessed a cold crushing strength of 3030 pounds per square inch, anda modulus of. rupture of 610 poundspersquareinch. Thebricksweighed approximately the same as those in the preceding example. They also exhibited still better resistance to load at high temperature, the brick having undergone but a slight subsidence at 2910 F., but not having failed in that temperature.

These brick also showed virtually no tendency either to expand or contract in burning or in re-' heating.

As evidencing the ability to use large amounts of olivine with relatively small amounts of magnesite, brick were made from the same materials using 60 percent of chrome ore, 30 percent of olivine and 10 percent of magnesite. Like brick C, these showed a slight tendency to expand in burning and in reheating, and they possessed still better mechanical properties, the cold crushing strength being 1630 pounds per square inch. The brick were better than brick C in resistance to load at high temperature as evidenced by the fact that the brick had not failed at 2910 F. under a load of 25 pounds per square inch, although they did show a slight subsidence at that temperature.

Brick made from chrome ore alone fail under a load of 25 pounds per square inch in the vicinity of 2500 F. The substantially increased temperature of failure of the brick made in accordance with this invention is, of course, an important feature because it permits the refractories to be used at higher temperatures and at higher loads. Tests of refractories within ranges constituting the present invention indicate that such improved resistance to load at high temperature is characteristic of all of the brick within the ranges stated, and further that in the higher ranges of magnesite and olivine content the ability to resist failure under a load of 25 pounds per square inch at temperatures-in the vicinity of 1600 F. and higher is .the rule. 1

The lack of appreciable volume change upon burning and reheating is also an important feature because it makes possible moreaccurate sizing and shaping of brick, and it reduces the strains to which the brick are exposed in heating and cooling. The reduced weight of the brick, as compared with those made from chrome ore alone, is also an advantage because refractory brick ordinarily are bought on a weight basis, and therefore the purchaser of brick made in accordance with this invention actually receives more bricks per ton than in the case of heavier bricks, which is advantageous to him. Likewise, handling and shipping weight is reduced and frame and foundation requirements are lessened.

Relative to the efiect of composition on weight and volume change, the weight of the brick provided by the invention ordinarily increases somewhat with magnesite and olivine toward the higherv ends of the ranges stated. And while all of the brick contemplated by the invention show virtually no volume change in drying and reheating, the greatest tendency to volume change is shown by the brick toward the lower ranges of magnesite and olivine content. We now believe that bricks of most compositions contemplated by the'invention will weigh between 165 to pounds per cubic foot. Generally speaking, therefore, the compositions of the brick made in accordance with this invention can be balanced according to particular conditions, those containing the highest amounts of olivine being the cheapest and this economic factor being'reached without sacrifice of refractory and mechanical properties, and even being accompanied in some instances by improved properties.

If the bricks are to be shipped unburned it will usually be desirable to add a temporary binder such as organic and other binders which are well known and used in the art.

It may be desirable in some instances also to add a mineralizer, of which a variety are known, including cryolite, fiuorspar, borax, and others. Usually the mineralizer will be present in rather small amount, for instance, not in excess of about 5 per cent.

In the practice of the invention a part or all of the chrome ore, alone or with olivine, may be pre-calcined, if desired, or, if desirable for any reason some or all of the constituents may be fused and crushed to prepare them for the manufacture of the refractories. Likewise, the chrome ore and the olivine, or either, may be subjectto the various purification and beneficiation treatments, such as magnetic separation, applied to the removal of-impurities or to improve the quality of the mineral.

In the practice of the'invention it will commonly be desirable to use a high quality magnesite. Such a magnesite is ordinarily of lowlime content, as indicated by the composition of the magnesite described hereinabove. In acopending application, Serial No. 26,520, filed June 13, 1935, we have shown that refractories containing forsteritic material may be improved for some purposes by the use. of magnesia-rich materials of relatively high-lime content, for instance, low grade magnesites containing, by way of example, from about 5 to 25 per cent of CaO in amounts such as to provide a composition containing lime between the limits of about 0.8 per cent and about 6.0 per cent. Since olivine is essentially a forsteritic material, it will be seen that the invention described and claimed in our aforesaid copending application may be applied 6 in practicing the present invention if that be desirable for any reason.

While the invention has been described chiefly with reference to the production of shaped and fired articles, nevertheless it contemplates also 10 the use of its refractory compositions as ramming mixes, for forming linings or the like, which are burned in place. Similarly, brick may be formed and dried, laid up in that condition in a furnace structure, and burned in place. Other modifications will occur to those skilled in the art.

Where the refractories are used for ramming and the likepurposes it may be desirable in some instances to add bonding agents, such as cements, an example being calcium aluminate cement of which one suitable type contains about 40 per cent of CaO, 42 per cent of A1203, 13 to 15 per cent 'of FeaOz, and 3 to 5 per cent of SiOz-plus- MgO. To avoid fiuxing of constituents of the refractory the amount of such cement should be quite low, e. g., 5 to 15 per cent.

According to the provisions of the patent statutes, we have explained the principle of our invention and have described what we now consider to represent its best embodiment. However, we 3 desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

We claim:

1. A refractory comprising more than about 50 per cent of chrome ore, at least about 10 per cent of olivine, and magnesite in amounts from onefourth to four times that of the olivine.

2. A refractory comprising from about 50 to 40 about 80 percent of chrome ore, olivine and magnesite in amounts substantially between the ratios 1:4 to 4:1, and the olivine constituting at least about 10 per cent of the refractory.

3. A burned refractory comprising from about to about 80 per cent of chrome ore, olivine and magnesite in amounts substantially between the ratios 1:4 to 4:1, and the olivine constituting at least about 10 per cent of the refractory.

4. A refractory consisting substantially of from 50 about 50 to 80 per cent of chrome ore, at least about 10 to. 40 per cent of olivine, and magnesite in amounts from about 5 to 40 per cent.

5. That method of making a refractory comprising preparing a batch containing at least about 50 per cent of chrome ore, at least about 10.per cent of olivine, and magnesite in amounts from about one-fourth to four times that of the olivine, forming the batch into shapes, drying the shapes, and firing them.

6. That method of making refractories comprising preparing a batch containing from about 50 to about 80 per. cent of relatively coarse chrome ore, at least about 10 per cent of olivine, and finely divided magnesite in amounts from about onefourth to four times that of the olivine, forming the batch into shapes, drying the shapes, and firing them.

'7. That method of making a refractory comprising preparing a batch containing from about 79 50 to about 80 per cent of relatively coarse chrome ore, at least about 10 per cent of finely divided olivine, and finely divided magnesite in amounts from about one-fourth to four times that of the olivine, forming the batch into shapes, drying the 1 shapes, and firing them.

8. A method in accordance with claim '7, said magnesite and olivine being finely divided so that a substantial percentage of each will pass about a '70-mesh screen.

9. A method according to claim 6. said magnesite being finely divided so that a substantial percentage will pass about a IO-mesh screen.

10. That method of making a refractory comprising crushing chrome ore to form relatively coarse granules, converting olivine and magnesite so that a substantial percentage thereof is in finely divided form, and mixing them to form a refractory containing from about 50 t0 80'per cent of said coarse chrome ore, finely divided olivine and magnesite in amounts substantially between the ratios 1:4 to 4:1, and containing at least about 10 per cent of olivine, and thereby providing a refractory material which upon being fired is characterized by substantial absence of volume change and by high temperature of failure under load.

11. That method of making a refractory comprising preparing a batch containing at least about 50 per cent of chrome ore, at least about 10 per centof olivine, and magnesite in amounts from about one-fourth to four times that of the olivine, sizing the batch to insure substantially the greatest density and strength in the fired rei'ractory, forming the batchinto shapes, drying the shapes, and firing them.

12. That method of making a refractory comprising preparing a batch containing at least about 50 per cent of chrome ore, at least about 10 per cent of olivine, and magnesite in amounts from about one-fourth to four times that of the olivine, grain sizing the batch, forming the batch into shapes, drying the shapes, and firing them.

13. That method of making a refractory comprising preparing a 'batch containing at least about 50 per cent of chrome ore, at least about 10 per cent of olivine, and magnesite in amounts from about one-fourth to four times that of the olivine, intermittent sizing the batch, forming the batch into shapes, drying the shapes, and firing them.

14. A refractory consisting substantially of from about 50 to 80 per cent of a spinel mineral containing a mixture of spinels of the group FeO.C12O3, MgQAlzOs and MgO.Cr2O3, at least about 10 to 40 per cent of olivine, and magnesite in amounts from' about 5 to 40 per cent.

15. That method of making a refractory comprising preparing a batch containing at least about 50 per cent of a spinel, containing a mixture of spinels of the group FeO.Cr2O3, MgOAizOz and MgO.CrzO:, at least about 10 per cent of olivine, and magnesite in amounts from about one-fourth to four times that of the olivine, forming the batch into shapes, drying the shapes, and firing them.

16. A refractory ramming mix comprising a mixture of refractory containing from about 50 to about 80 per cent of chrome ore, and olivine and magnesite in amounts substantially between the ratios 1:4 to 4:1, the olivine constituting at least about 10 per cent of the refractory; and hydraulic cement in an amount between about 5 to 15 per cent based upon the weight of the refractory.

17. A refractory ramming mix comprising a refractory composition consisting substantially of from about 50 to 80 per cent of chrome ore, about 10 to 40 per cent of olivine, and about 5 to 40 per cent of magnesite; and from about 5 to 15 per about to 4 times that of the olivine; and 1137- draulic cement in an amount from about 5 to 15 per cent based upon the weight of the refractory.

19. A ramming mix according to claim 17, said hydraulic cement being an alumina cement.

20. A ramming mix according to claim 18, said hydraulic cement being an alumina cement.

FREDERIC A. HARVEY. RAYMOND E. BIRCH. 

